1. Field of the Invention
This invention relates to a line sensor for color reading which is a reading means for reading a color manuscript for each line through color separation and a color image reading device.
2. Description of the Prior Art
In the prior art, in facsimiles, digital copying machines or electronic file systems, it has been practiced to read a manuscript image information while scanning each line. More specifically, a line sensor is constituted by arranging one-dimensionally photosensors having light-receiving portions with adequate sizes and, by moving said line sensor relative to the manuscript in the direction substantially perpendicular to the lengthwise direction of said line sensor, the manuscript is successively read with the size of the manuscript surface corresponding to the size of the light-receiving portion of the photosensor as one image element.
And, in recent years, it is gradually becoming demanded to read colors.
In this context, as the general method for performing reading by use of a line sensor, there is the method in which CCD is used as the photosensor and image formation of the manuscript surface is effected on CCD by use of a reduction image forming optical system. However, in such a reading method, it is required to take a longer optical path length of the image forming optical system as the line sensor is made longer, whereby the device will disadvantageously become enlarged. Also, due to the small area of each light-receiving element of CCD, it is lower in sensitivity and therefore it is required to intensify the manuscript illumination light source or to retard reading speed. In particular, CCD is lower in sensitivity to blue color and hence the problem remains in the case of color reading.
Another method for performing reading by use of a line sensor, there is the method in which the so called planar type photoconductive type photosensor as the photosensor and an erecting equimultiple image forming optical system such as rod lens array, etc. is used (namely the method according to the so called contact type line sensor). According to this method, the optical path length of the optical system may be smaller, whereby miniaturization of the device can be realized.
A planar type photoconductive photosensor can be constituted by arranging a pair of electrodes by provision of a gap constituting at least a part of the light-receiving portion on a photoconductive layer such as of chalcogenide, CdS, CdSSe, amorphous silicon (hereinafter written as a-Si), etc. Particularly, a planar type photoconductive photosensor by use of a-Si is good in light response speed and also large in output photocurrent, and therefore a good line sensor can be prepared therefrom. Also, a-Si materials, which are free from pollution and for which silicon technology such as the plasma CVD technique, photolithographic patterning technique, etc. may be employed, are also excellent in productivity. Further, as shown in FIG. 1, an a-Si photosensor has a spectral sensitivity which is approximate to the specific visionary sensitivity, thus having a great adaptability for a color sensor. In other words, the output photocurrent values of the photosensor relative to the same energy light each of 10 .mu.W/cm.sup.2 at blue 450 nm: green 550 nm: red 650 nm are 2: 3.5: 3, thus being flat in spectral sensitivity, which is preferred for a color photosensor.
As the method for performing color reading in such a contact type line sensor, it has been known to arrange, for example, linear light sources of three colors of red, green and blue at the light source for manuscript illumination in parallel to the line sensor and red successively the respective color signals with one line sensor by lighting successively the light sources of respective colors. Whereas, under the present situation, there is no small scale blue light source which can give sufficient dosage as a linear light source, whereby miniturization of the device cannot sufficiently be effected. Also, according to this method, since reading of the three colors is performed under the same optical system arrangement condition for the same image element of the manuscript, there is involved the problem that a considerable time is required for reading.
On the other hand, another method for performing color reading in a contact type line sensor comprises arranging, for example, color filters of red, green and blue at positions corresponding to the light-receiving portions of the respective photosensors, grouping the three colors of red, green and blue in the arrangement of the color filters, dividing one image element of the manuscript surface corresponding to the three photosensors of one group into three divisions and reading the color signal obtained from each division as a typical value of each color signal of the image element (psuedosame point reading). However, since this reading method reads the color signals of respective colors from substantially different manuscript surfaces, the quality of the image read will be lowered, and thus there may be involved the problem of causing lowering in image quality such that a monochromatic brim may appear on a black letter image in the outputted image synthesized from the color signals of three colors obtained.
On the other hand, as another requirement, it is desired for an image sensor that the reading time should be as short as possible and the energy required for illumination of an object as small as possible. For this purpose, the image forming optical system to be used is required to be as light as possible.
However, a light optical system to be used in a colored system has a problem. That is, in a light image forming optical system, the image forming position tends to differ frequently depending on the wavelength of light.
Therefore, when an image sensor is provided at the image forming position of a light with a specific wavelength (e.g. a green light), resolution will markedly be lowered relative to the lights with other wavelengths (e.g. red light, blue light). For this reason, it is necessary to use a dark image forming optical system with little of such a tendency or prepare separate image forming optical systems for lights with respective wavelengths, thus involving great drawbacks in aspects of high speed reading, compaction, cost, etc. of an image sensor.
Accordingly, some the present inventors, with others, have proposed a color image reading method and device which can cancel the above drawbacks (U.S. Pat. application Ser. No. 505,270, filed June 17, 1983, now U.S. Pat. No. 4,558,357, The color image reading device according to this proposal (hereinafter referred merely as the prior art device) comprises a plurality of line image sensors being provided corresponding to the lights within wavelength ranges corresponding to specific colors, said image sensors in respective lines being arranged at the image forming positions where the resolution of the light within said wavelength ranges can be maximum.
FIG. 2 is a schematic illustration of the construction of the prior art device.
In this figure, the manuscript 202 moving in the direction of the arrowhead A is illuminated by a light source 204, and the image on the illuminated will manuscript 202 form an image on a substrate 208 through the ommateal optical system 206. On the substrate 208, photosensor arrays 210, 212 and 214 are formed, and on the respective photosensor arrays are provided a red filter 216 with wavelength range corresponding to red color, similarly a green filter 218 and a blue filter 220, respectively. Also, the photosensor arrays 210, 212 and 214 are arranged at the positions where the respective color components of the image on the manuscript 202 at the positions L1, L2 and L3 form images, respectively.
In such a construction, the images at the respective portions on the manuscript 202 will be read, while moving toward the direction of the arrowhead A, at the position L1 of the red component by the photosensor 210, at the position L2 of the green component by the photosensor 212 and at the position L3 of the blue component by the photosensor 214, respectively. Accordingly, the respective color components of the manuscript 202 enter accurately the respective photosensor arrays, whereby reading of high resolution can be effected.
However, in such a device of the prior art, it is difficult to ensure the wiring space for the photosensor array 212 positioned at the center, particularly difficult in the case of a matrix wiring. Further, if the three photosensor arrays are formed on the same substrate 208, the generation probability of rejected sensors per one substrate will be increased with an increase of the number of the photosensors, whereby there is involved the problem of lowering in production yield, for the number of the color signals to be separated and arranged in parallel each other on one and the same substrate, and a color filter for color signal separation directly disposed on the light-receiving portion of each of said line sensors.